This invention relates to a flight management system for an aircraft, and more particularly, to an apparatus and method for wind forecast error compensation for 4-D guidance in a flight management system, thereby improving arrival time performance. Controlling an aircraft to arrive at a waypoint at a prescribed time is sometimes referred to as 4-D guidance.
An aircraft flight management system (FMS) performs many functions in controlling the operation of an aircraft, including prediction of arrival time. A flight management system needs to make speed adjustments in order to achieve a required time of arrival (RTA) at a designated waypoint or destination. Such adjustments are based on repeated predictions (fast-time simulated flight) that use a wind forecast model. Wind forecasts always have errors.
Aircraft are controlled to fly air speed, rather than ground speed, because aerodynamic characteristics and operating limits are related to air speed. However, flight time is a function of ground speed, which is the sum of the horizontal component of true air speed (TAS) and tail wind (the projection of the wind vector along the horizontal flight path). FMS predictions use a model for estimating the wind, but even if this model has the correct wind over the whole flight plan at any given time, by the time the real aircraft arrives at points downstream the actual wind has undoubtedly changed. The result is that repeated speed adjustments, based on revised periodic predictions using forecast with error, tend to grow exponentially as the aircraft approaches the destination. Compounding this problem is the 250 (knots-kn) CAS speed limit that is normally in effect below 10,000 ft in USA which prevents increasing speed to compensate for unexpected head wind. Current FMS predictions use a blend of the known wind at the aircraft position with the forecast wind at the prediction positions for some distance ahead of the aircraft. This method offers a some improvement but is inadequate for improving accuracy in arrival time.
The present invention provides compensation for errors in the forecast model in order to improve the arrival-time performance.